Preheat oven to 400 degrees F. Place a sturdy cast iron skillet in the oven to preheat as well.
To prep the chicken, use kitchen shears to remove the backbone out of a whole roaster chicken. Rub the chicken with olive oil and season the chicken well with lemon pepper and kosher salt. You can prep the chicken up to 24 hours in advance up to this point.
When ready to roast, slice a lemon and lay the slices in the middle of a baking dish. Place the chicken in the dish, skin-side up.
Prep the veggies by halving the potatoes and cutting the carrots into about 1-inch pieces. Then toss with olive oil and salt and pepper. Pour the veggies around the edges of the spatchcocked chicken.
Add a piece of clean foil to the top of the chicken and place the hot cast iron skillet on top of the foil to press the chicken down and apply direct heat to the top. Be careful!
Move the entire baking dish with the cast iron skillet to the oven. Roast for 15 minutes.
Remove the cast iron skillet and foil from the chicken. Add fresh thyme to the dish (optional). Return to oven and roast for another 30 minutes.
Chicken should be cooked through at this point but you should always check it with a meat thermometer to ensure it’s a safe temperature. The chicken should be 165-170 degrees F. in the thickest part of the thigh and breast.
Remove the chicken from the oven, let it rest for a few minutes before slicing and serving!